The present invention relates to a position control technology of a gantry in an X-ray CT (Computed Tomography) that performs a scan to take a tomogram of an object to be inspected while moving the gantry.
An X-ray CT system has been known whose gantry is mounted on a bogie moving on rails by means of wheels and which performs a scan of an object to be inspected while moving the gantry.
Generally, in a rail, its cross section is formed convexly and the convex portion becomes a rail tread surface. A groove corresponding to the width of the rail tread surface is formed on a wheel tread surface along the direction of rolling of the wheel (that is, the wheel tread surface is formed concavely). This groove is engaged with the rail convex portion to enable the wheel to move along the rail to thereby prevent the wheel from coming off the rail.
In order to roll the wheels on the rails smoothly, a gap of about 0.5 mm to 1 mm is usually provided between the rail tread surface and the groove of the wheel tread surface. Four wheels of the bogie are machined so that they have the same diameter, but there is a little difference in size between them and the difference in size causes the bogie to move not in a straight line but in a little curved line to the left and right side. As the bogie moves forward, the bogie is deviated-in the lateral direction to narrow the provided gap gradually and when one side of the groove comes into contact with one side of the rail, the bogie can not move further in the deviated direction but is moved straight in the direction of length of the rail forcibly by the groove and the rail.
Next, when the bogie moves in the reverse direction, the bogie is deviated reversely in the lateral direction and when the opposite side of the groove comes into contact with the rail, the bogie is forcibly moved straight in the direction of length of the rail. That is, the position of the bogie when the bogie moves forward is deviated in the lateral direction from the position of the bogie when the bogie moves backward. As a result, there is presented a problem that the rotational center of scan rotation is deviated.
Moreover, while an rotary encoder is mounted on the shaft of a driven wheel in the related art to detect the position of the bogie by detecting the rotation of the wheel, as described above, because the position of the bogie when the bogie moves forward is deviated in the lateral direction from the position of the bogie when the bogie moves backward, the wheel does not move in the same locus when the bogie moves forward and when the bogie moves backward. For this reason, there is presented a problem that when the bogie stops at a certain position in the direction of length of the rail, the detection result of position of the bogie is different depending on the direction of movement of the bogie before the stop of the bogie. A problem that a large error is caused in detecting the position of the bogie results in a deviation in a scan position in the direction of length of the rail.
Moreover, the rotation of the rotary encoder depends on the size of the wheel. For this reason, there is also presented a problem that since the detection accuracy of movement distance of the bogie in the direction of length of the rail depends on the machining accuracy of the size of the wheel, a desired position detection accuracy can not be kept.
Further, there is also presented the following problem: while the position of the bogie when the bogie moves back and forth can be recognized by the detection of a limit switch at a retracted position of the gantry, it is necessary for an operator to press a button provided on the gantry to set a zero slice position that is the base of scan (scan base position), which makes operability worse. In particular, this includes a problem that even if changing the scan base position is not required because the predetermined scan base position exists, it is necessary for the operator to input the scan base position manually, which makes operability worse and tends to make an operating miss or to cause a position error.
Still further, safety must be sufficiently secured when the gantry is moved.